According to the article you listed, the article differentiates content from utterance, proposition from sentence:
The signifier is the constitutive unit of the symbolic order because it is integrally related with the concept of structure.
So, what Lacan is on about is the distinction most philosophers make between the content, the semantics, the proposition, the concept, or the message (all variously used) with the utterance, the syntax, the sentence, the encoding, or the medium, resp. Our brains express noises, our minds express ideas. It's fascinating, for instance, that voice recognition as a technology took a long time to improve because look at the structure of a sound wave, words in a sentence aren't broken up physically. They're part of a continuous sound wave. Words, therefore, are a logical imposition on sound constructed by the mind in the same way wavelengths of EM energy in the visual portion of the spectrum aren't actually colors.
Since Charles S. Pierce and Ferdinand de Saussure and the introduction of semiotics, many thinkers have struggled to come to terms with the duality that is inherent in linguistic artifacts. On the one hand, words can be written and spoken, but they seem to depend on a physical medium. No one, for instance, has ever produced scientific evidence of words or ideas transcending physical reality. In philosophy, one might say that ideas supervene on the medium of their expression, and words are no different.
Another model that has a lot of currency is Ogden and Richard's triangle of reference. Lacan glommed onto, according to the article, a different semiotic emphasis:
Thus for Lacan language is not a system of signs -- as it was for Saussure -- but a system of signifiers.
And that's because he fundamentally disagrees with Saussure:
Whereas Saussure argues that the signifier and the signified are mutually interdependent, Lacan states that the signifier is primary and produces the signified.
So, whereas this article states Saussure sees the interplay as bidirectional, Lacan takes the signifier as primary. And why?
It is these meaningless indestructible signifiers which determine the subject; the effects of the signifier on the subject constitute the unconscious, and hence also constitute the whole of the field of psychoanalysis.
So, my interpretation of this statement is that fundamental to the construction of the ego is the use of signifiers. Think about your personal life. How much of your own self-description occurs in language? I suspect nearly all of it. Therefore, psychoanalysis is the art, on this view, of getting a subject or patient to express themselves to give a window into the mind, particularly unconscious mind and the processes associated with. That doesn't seem to controversial by today's standards, the notion that talk therapy provides a window into a patient's mental health, anyway.
In the article:
The signifier is the constitutive unit of the symbolic order because it is integrally related with the concept of structure.
This is just the principle of compositionality in other words. A word like 'ellucidate' literally decomposes into Latin prefix and stem that means "draw into the light" capturing the extended metaphor of seeing as an analogy to understanding. A passage therefore is understood by the mind processing language and reducing it to signifiers that are primitives, whatever they may be, and why they're interconnected and regress and so on is that words are generally defined in terms of other words. For instance, every word in a dictionary entry for a definition seem to exist as their own entries. Hot relies on cold, up relies on down, temperature relies on measurement, and so on. Thus, we see the typical psychoanalytical approach that there might be great significance in the analysis of a patients words, and that the meanings that you extract from a patient's statements is complex and interconnected:
Not only can units of language smaller than words (morphemes and phonemes) or larger than words (phrases and sentences) also function as signifiers, but so also can non-linguistic things such as objects, relationships and symptomatic acts.
Thus, morphemes and phonemes comprise lexemes, lexemes comprise phrases and sentences, and those are part of passages which provide context. And to boot, meaning occurs not only with the primitive, but the composition of the parts. Hence, psychoanalysis is part of the movement in philosophy that is recognized as the linguistic turn. One can administer leeches to cure the brain, or one can have a conversation with someone who is mentally ill and try to construct their perspective (their subjective view, the subject, the phenomenological experience) and find a solution to mental maladies. Believe it or not, this is actually a rather revolutionary view since the history of mental illness often just presumed physical disease or demonic possession or in ancient times, apparently, a wandering uterus in the body. Therefore, Lacan, in the tradition of Freud was advocating an analysis of language to get at the subconscious mind to infer the nature of the impairment so that it might be corrected.
